This invention relates to industrial control systems for controlling the operation of machinery, such as motors and actuators, and in particular to a method and system for providing a responsive and consistent jogging of a controlled machine in an industrial control network.
An industrial control system may include a central industrial controller connected via a communications network to one or more spatially remote input-output (I/O) devices in turn connected to the controlled machinery. Industrial controllers are special-purpose computers processing input data received from sensors on the controlled machinery or from operators, to produce output data transmitted to the controlled machinery. The communications networks relay this data to and from the industrial controller. Communications networks used for this purpose include the well-known protocols of Ethernet, DeviceNet, ControlNet, Fire Wire and Field Bus. The I/O devices provide an interface between the network signals and the control machines.
Often, the operator of a machine controlled by such a system needs to jog or incrementally advance the machine. This is common in robotics or motion control systems where the operators must make slight position adjustments to perform a specific task. Under conventional practice, in order to jog a machine, the operator presses a button on a control panel such as a human/machine interface (HMI) to create a xe2x80x9cmachine startxe2x80x9d message. This message is sent through the network to the industrial controller. The industrial controller processes the machine start message with the control program (which may for example, ensure that the machine is not in a xe2x80x9cstop modexe2x80x9d in which jogging is prohibited) then transmits the message again through the network to an I/O device. The I/O device converts the message to electrical signals compatible with the machine and the machine is activated. Then, when the operator releases the button, a xe2x80x9cstop messagexe2x80x9d is sent via the same or similar path to deactivate the machine. The operator viewing the machine and the dynamics of the jogging operation may make very fine and accurate adjustments so long as the amount of jogging is consistent from jog to jog. Unfortunately this is not always the case.
As mentioned the jog commands may be received by the industrial control system through one or more human/machine interfaces (HMI""s) which may employ a matrix of membrane or other switches having predefined designations. It is normal to read such switches in a scanning process that scans through the switches, thereby reducing the amount of wiring needed to connect the switches to internal control circuitry. The speed of this scanning may be relatively slow and thereby may introduce an unpredictable delay in the generation of a switch signal depending on when during this scan the switch is pressed.
Once the jog signal is created, it must be transmitted over the network. In order that the communication of data is timely and predictable, most industrial control networks include a protocol in which the data is assigned to specific network slots, a network slot being a predefined time interval during which communication occurs on the network. Input and output data is queued until the network slots at the scheduled time are available. This queuing introduces a bounded but unpredictable delay in the transmission of the data depending on the amount of message traffic and the priority of the message. In order to reduce this delay, time critical messages may be assigned a high priority on the network, where high priority messages obtain use of the shared communication media in preference to low priority messages. Normally, however, even high priority message experience some delay both because of the competition between high priority messages and because the proportion of message carrying capacity of the communications network allocated to high priority messages is limited to ensure that low priority messages will also be guaranteed transmission.
The data received by the industrial controller is processed during a cycling xe2x80x9cscanxe2x80x9d of a stored control program during which the industrial controller examines received inputs reflecting the status of the controlled process and changes the series of outputs that will ultimately be transmitted on the network to control the industrial process. Depending on when the data is received with respect to the scanning of the control program, a bounded but unpredictable delay will occur before the data is processed by the control program.
Additionally, individual components of the control system operate asynchronously in relation to each other. This exacerbates the aforementioned delay problem and the machine will not be activated immediately. This delay can result in the operator inputting multiple signals, believing that the machine did not respond to the initial input or being frustrated as the machine moves slowly up on the target then jumps beyond it as a result of inconsistent jogging action.
The present invention provides a method and system for jogging equipment controlled by the industrial control network that overcomes the problems of the prior art. In particular, the present invention provides consistent jogs by processing special jog messages defining fixed-length jog pulses at the controlled equipment. Also, the present invention minimizes network delay effecting the jog by processing the jog message at the controlled equipment drive unit.
Specifically, the method and system of the present invention include an industrial control system for controlling the operation of a machine having a communications network connecting a jog button to at least one output device, which may be separate from or incorporated into the machine. The communications network introduces a variable delay in the transmission of messages. In response to a pressing of a jog button by a human operator who is observing the operation of the machine, a predefined jog message is transmitting on the network. The jog message includes a jog duration period and an address corresponding to the machine. The jog message is received at the output device corresponding to the machine, which reads the message and turns on the corresponding machine, waits for the jog duration period to expire and then turns off the machine. The time the machine is turned on is the same as the jog duration period, despite the delay introduced by the communications network.
The present invention thus provides the object and advantage of allowing practical jogging of machines on a network-based control system. The invention provides responsive jogging much like direct wired control systems, but at the lower installation cost and simplicity of a network system. Reducing the time delay from when the jog button is depressed to the start of the jog gives the operator a more intuitive feeling of control. Similarly, eliminating the variation in jog length improves the accuracy of positioning and advancement the machine.
Another aspect of the invention is that the industrial control system includes a centralized industrial controller executing a control program and communicating with the communications network to send and receive messages thereon in accordance with the control program. Alternatively, the industrial control system may include multiple distributed industrial controllers executing portions of a control program and communicating with the communications network to send and receive messages thereon in accordance with their portions of the control program.
The present invention provides another object and advantage in that is operable in central and distributed control systems. The specialized jog messaging of the present invention allows selective bypassing of the controller so that the controller may be bypassed for jog functions but utilized for other functions.
Yet another aspect of the invention is that the jog button may be a common mechanical, capacitive or photoelectric switch. Or, the jog button may be a part of a human-machine interface having a plurality of buttons scanned in sequence. In each case, the jog duration may be pre-selected by the operator.
The present invention thus provides another object and advantage by allowing responsive and accurate jogging of a network machine with switches such as membrane and capacitive switches having relatively slow response times.
Still another aspect of the present invention is that the jog message may be received at a industrial controller after being transmitted on the network, in which case the industrial controller executes a stored program to read the jog message at a periodic scan. The industrial controller analyzes the state of the network to determine whether the machine at the network address should be jogged. If so, the jog message is transmitted on the network to the output device corresponding to the machine at the network address of the jog message.
The present invention provides the further object and advantage of allowing a control program executed by an industrial controller to supervise whether jogging can occur.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.